MOSCOW, Jan 28 (Reuters) - Russia's state arms trader declined to say on Thursday whether it would go ahead with the sale of S-300 anti-aircraft hardware to Iran, but made clear it did not view the systems posing a threat if Iran obtained them.

The possible sale of the S-300s, which could protect Iran's nuclear facilities against air strikes, is a sensitive issue in Russia's relations with the United States and Israel, which have pressed Moscow not to proceed with the deal.

"I just don't quite understand why supplies of the S-300 system to Iran trouble you so much," Anatoly Isaikin, the head of Rosoboronexport, replied after being repeatedly asked about the deal at a news briefing.

"This is purely a weapon of defence, not attack," he added. "This weapon cannot pose any threat to any neigbours, close or distant."

Russia is under intense Western pressure to distance itself from Iran in the dispute over Tehran's nuclear programme, but has refused to rule out the delivery of the S-300 system.

In Jerusalem, an Israeli official told Reuters Russia had yet to ship the S-300's "main systems" -- such as radars and interceptor missiles -- to Iran.

Analysts say the S-300 could help Iran thwart any attempt by Israel or the United States to bomb its nuclear facilities from the air.

Isaikin said journalists must be guided by statements made by other Russian officials last year. "From my part, I just want to say I have nothing to add to these statements."

Last year, Russian officials said Iran was not under international sanctions that would restrict its purchases of defence systems, but left it unclear whether any parts of the S-300 had actually been delivered.

Isaikin said there was no formal obstacle to weapons sales to Iran, which he described as Moscow's "long-term partner".

"For all these years there has been not a single case when it breached its contractual obligations, including by handing over Russian weapons to a third country," he said.

SOARING ARMS EXPORTS

Isaikin said that Rosoboronexport's weapons exports this year would be no lower than last year's $7.4 billion, and that its portfolio of current export orders exceeded $34 billion. He said the portfolio had expanded by $15 billion last year.

Isaikin said the state arms trader's exports had increased by more than 2.4 times between 2001 and 2009. Military aviation exports account for roughly a half of all arms sales, he said.

Rosoboronexport says it accounts for around 80 percent of all Russian arms exports, with weapons sold to more than 70 countries. Its main export destinations are India, Algeria, China, Venezuela, Malaysia and Syria.

The truck-mounted S-300PMU1, known in the West as the SA-20, can shoot down cruise missiles and aircraft. It has a range of 150 km (90 miles) and travels at more than two km per second.

Washington has sought pledges from Russia for tougher sanctions against Iran over its nuclear energy programme, which the West suspects is intended to produce nuclear weapons. Tehran denies any such intention.

Israel, which is thought to have the Middle East's only atomic arsenal, has hinted it could attack Iran in an effort to prevent it from obtaining nuclear weapons.

Iran has threatened to retaliate for any attack by firing medium-range missiles at Israel. (Additional reporting by Dan Williams in Jerusalem; writing by Amie Ferris-Rotman and Dmitry Solovyov; editing by Noah Barkin)